1. Field of the Disclosure
The present application is related to electronic devices, and in particular to real-space charge-transfer devices and methods based on real-space charge transfer devices.
2. Related Art
Real-space charge-transfer devices, such as a Gunn diode, can be used to generate microwaves or millimeter waves. Such a device has an active region between an anode and a cathode that can be manufactured using a semiconductor material, such as a compound semiconductor in the case of a Gunn diode. It is the case with such devices that their electron mobility is large in a low electric field (several thousands of cm2 N-sec) and that in response to being exposed to a sufficiently large electric field their electron mobility is decreased as accelerated electrons transit to a band of large effective mass. This decrease in mobility in high electric fields causes a negative differential mobility within their active region that is characterized by the generation of a p-n junction domain that transits across the active region of the device, from the cathode side to the anode side. This domain is referred to as a Gunn domain in Gunn diodes. Once the p-n junction domain completes its transit across the active region of a device, another p-n junction domain is generated and begins its transit across the device. FIG. 1 illustrates a current-time graph illustrating the vibrating current at the anode of a Gunn diode that is the result of this phenomenon. As illustrated, the duration of the Gunn domain is represented by the time labeled DGd, and the period of the vibrating current of the Gunn Domain that results in the normal Gunn oscillation fG is represented by the time labeled PGd,osc.
The Oscillation frequency of a standard Gunn diode can be determined from the transit distance L of the domain, e.g., the length of the Gunn diode's active region, and the average drift velocity Vd of the electrons in the active region using the equation: ft=Vd/L. Thus, the energy relaxation time of the device, which consists of the time needed for the electron to increase and decrease energy at Γ valley, and the length of the device primarily determine the upper limit of the oscillating frequency in the millimeter wave range. For example, the relaxation time constant of GaAs is such that the upper limit of the oscillating frequency for a Gunn diode is between 60 and 70 GHz (gigahertz).
Efforts to increase the upper frequency limits of Gunn diodes include using compound semiconductor materials having faster relaxation time constants. In addition, the distance of transit has been short, e.g., 1 to 2 μm (micrometers).
In order to implement such efforts, measures have been taken with conventional Gunn diodes for millimeter waves such as employing a vertical diode structure having an anode and cathode at opposing surfaces, to use elements including the active layer of extremely small sizes, having diameters of approximately several tens of μm.
The use of the same reference symbols in different drawings indicates similar or identical items.